The present invention relates to a photodetector and more particularly relates to a high-speed photodetector with the capacitance at its pad portion reduced by forming a mesa-shaped light-absorbing layer on part of a semi-insulating semiconductor substrate and an electrode pad an another part thereof, respectively.
A photodetector for use in fiber optics communication, which exhibits a photosensitivity to some incident radiation with a long wavelength ranging from 1.3 xcexcm to 1.55 xcexcm, may be implemented typically as a pin photodiode made of InGaAs and InP compound semiconductors. A pin photodiode of this type often has its response speed restricted by a CR time constant, which is a product of the capacitance of the photodiode and a load resistance. Accordingly, to increase the response speed of a pin photodiode, the photodiode needs to have a reduced capacitance.
And to reduce the capacitance of a photodiode, not only the junction capacitance but also the capacitance associated with its electrode pad should be reduced as well. In high-speed photodetectors (or photodiodes) of today, in particular, the photodiode section thereof has a much smaller size. Accordingly, a ring electrode (typically with a diameter of about 35 xcexcm) formed on the photodiode section is now smaller in size than an electrode pad (typically with a diameter of about 80 xcexcm) extended from, and disposed near, the ring electrode. For that reason, the capacitance of the electrode pad has a considerable effect on the response speed of the photodiode. In a known structure specially designed to reduce the pad capacitance, a thick insulating film of polyimide is interposed between the electrode pad and a semiconductor layer. However, to further reduce and almost eliminate the pad capacitance, another known structure includes: a mesa-shaped light-absorbing layer on part of a semi-insulating semiconductor substrate; and an electrode pad on another part thereof on which the light-absorbing layer does not exist.
A photodetector with such a structure is disclosed, for example, in Japanese Laid-Open Publication No. 5-82829. FIG. 9 schematically illustrates the structure of the photodetector disclosed in this publication.
The photodetector 500 shown in FIG. 9 includes photodiode mesa 505 and pad mesa 506 that are formed on a semi-insulating InP substrate 501. More specifically, the photodiode mesa 505 includes n+-InP, nxe2x88x92-InGaAs and n-InP layers 502, 503 and 504, which are stacked in this order on part of the substrate 501. On the other hand, the pad mesa 506 also includes the n+-InP, nxe2x88x92-InGaAs and n-InP layers 502, 503 and 504, which are stacked in this order on another part of the substrate 501 where the photodiode mesa 505 does not exist. A pad electrode 511 is formed on the upper surface of the pad mesa 506.
The photodiode mesa 505 further includes a p+-type doped region 507 that has been formed by heavily doping part of the n-InP layer 504 with a p-type dopant so that the dopant reaches the InGaAs layer 503 as a light-absorbing layer. And an insulating film 510 of SiN has been deposited over the substrate 501. A p-side electrode 508 is formed on the insulating film 510 and is electrically connected to part of the doped region 507. An n-side electrode 509 is also formed on the insulating film 510 but is electrically connected to part of the n-InP layer 504 where the doped region 507 does not exist. The p-side electrode 508 on the photodiode mesa 505 is connected to the pad electrode 511 on the pad mesa 506 by way of an interconnect 512 that has been formed on the insulating film 510.
In the photodetector 500 shown in FIG. 9, part of the n+-InP layer 502, which existed between the photodiode and pad mesas 505 and 506 originally, has been removed completely to electrically isolate the photodiode and pad mesas 505 and 506 from each other. The n+-InP layer 502 will be herein called a xe2x80x9csemiconductor conductive layerxe2x80x9d. However, the semiconductor conductive layer 502 and semiconductor substrate 501 are both made of InP, and it is difficult to etch away that part alone as intended. For that reason, in the known photodiode 500, the semiconductor conductive layer 502 is etched rather deep and the surface of the substrate 501 is also etched away partially to remove that part of the semiconductor conductive layer 502 located between the photodiode and pad mesas 505 and 506 completely. As a result, the photodiode and pad mesas 505 and 506 can be isolated electrically, but the respective heights of the mesas 505 and 506 as measured from the surface of the substrate 501 are higher than the originally designed ones.
The higher the photodiode and pad mesas 505 and 506, the harder it is to form the interconnect 512 and bridge these mesas 505 and 506 together as designed. This is because where the mesas 505 and 506 are so high, part of the interconnect 512 located around the corner between the photodiode or pad mesa 505 and 506 and the substrate 501 easily peels off. That is to say, to form the interconnect 512 more reliably, the mesas 505 and 506 should preferably have their heights reduced. In the known photodetector 500, however, the heights of the photodiode and pad mesas 505 and 506 exceed the minimum required ones to completely isolate these mesas 505 and 506 electrically from each other.
In addition, the photodetector 500 shown in FIG. 9 also has a non-negligibly large interconnect capacitance. In the photodetector 500, the pad electrode 511 and part of the interconnect 512 on the substrate 501 create no parasitic capacitance. However, another part of the interconnect 512 on the photodiode mesa 505 does create some interconnect capacitance. Where the photodetector 500 should operate at a high speed with the area of the doped region 507 minimized, this interconnect capacitance is non-negligibly large compared to the junction capacitance thereof. Particularly when the insulating film 510 located between the interconnect 512 and the n-InP layer 504 (which will be herein sometimes called a xe2x80x9cwindow layerxe2x80x9d) is made of a single SiN layer, the interconnect capacitance increases noticeably. The reason is as follows. Firstly, the SiN layer should be thin enough because cracks would be formed easily otherwise. Also, an SiN film has a higher dielectric constant than that of any other insulating film (e.g., SiO2 film).
It is therefore an object of the present invention to provide a photodetector that can be mass-produced easily.
Another object of this invention is to provide a high-performance photodetector with an optical filtering function.
Still another object of the invention is to provide a high-speed photodetector with a reduced interconnect capacitance.
An inventive photodetector includes semi-insulating semiconductor substrate, semiconductor conductive layer, light-absorbing layer, wide bandgap layer and doped region. The conductive layer has been formed on a surface region of the substrate and has electrical conductivity. The light-absorbing layer has been formed on the conductive layer and absorbs light that has been incident on the photodetector. The wide bandgap layer has been formed on the light-absorbing layer and has a bandgap wider than that of the light-absorbing layer. And the doped region has been defined in the wide bandgap layer by doping part of the wide bandgap layer with a dopant that reaches the light-absorbing layer. In this photodetector, the conductive layer has etch susceptibility different from that of the substrate.
In the photodetector according to a first aspect of the present invention, the conductive layer has etch susceptibility different from that of the substrate. Accordingly, by using an etchant (e.g., an etchant containing hydrochloric acid) that etches the conductive layer selectively with respect to the substrate, the conductive layer can be etched just as intended. That is to say, the etch process may be stopped as soon as the surface of the substrate is exposed. Thus, the etch process is controllable much more easily and there is no need to remove the uppermost part of the substrate. Accordingly, the mesas do not increase their heights too much. As a result, the interconnect can be formed easily and the photodetector of this type is mass-producible. Also, where a semiconductor multilayer structure (i.e., photodiode mesa), including the semiconductor conductive, light-absorbing and wide bandgap layers, has stepped side faces, a photoresist pattern, determining the shape of the interconnect, can be defined better compared to a mesa with no stepped side faces. Thus, the interconnect can be formed even more satisfactorily.
In one embodiment of the present invention, the substrate, conductive layer, light-absorbing layer and wide bandgap layer may be made of InP, InGaAsP, InGaAs and InP, respectively.
In another embodiment, InGaAsP as a material for the conductive layer may have an absorption edge longer than 0.93 xcexcm and shorter than 1.55 xcexcm.
In still another embodiment, the conductive layer may be an n-type semiconductor layer, the dopant may be a p-type dopant and the light-absorbing layer may function as an intrinsic layer of a pin photodiode. And the photodetector may further include: an n-side electrode, which makes an electrical contact with the conductive layer; and a p-side electrode, which makes an electrical contact with the doped region.
In yet another embodiment, a semiconductor multilayer structure, including the semiconductor conductive, light-absorbing and wide bandgap layers, may have been formed on said surface region of the substrate. A second semiconductor conductive layer may have been formed on another surface region of the substrate and may be electrically isolated from the conductive layer included in the multilayer structure. A pad for use to electrically connect the photodetector to an external unit may have been formed on the second conductive layer. And the pad may be electrically connected to the doped region that has been defined in said part of the wide bandgap layer in the multilayer structure.
In this particular embodiment, a ring electrode with an opening at the center thereof has preferably been formed on the doped region. And the ring electrode is preferably connected to the pad by way of an interconnect that has been formed on an insulating film. The insulating film preferably covers the surface of the multilayer structure.
Alternatively or additionally, the semiconductor conductive, light-absorbing and wide bandgap layers, making up the multilayer structure, have preferably been stacked one upon the other to make a level difference exist between each of these layers and an adjacent one of the layers.
Another inventive photodetector includes semi-insulating semiconductor substrate, semiconductor conductive layer, light-absorbing layer, carrier barrier layer, wide bandgap layer and doped region. The conductive layer has been formed on a surface region of the substrate and has electrical conductivity. The light-absorbing layer absorbs light that has been incident on the photodetector. The carrier barrier layer has been formed between the conductive and light-absorbing layers to prevent carriers, created in the conductive layer, from diffusing and entering the light-absorbing layer. The wide bandgap layer has been formed on the light-absorbing layer and has a bandgap wider than that of the light-absorbing layer. And the doped region has been defined in the wide bandgap layer by doping part of the wide bandgap layer with a dopant that reaches the light-absorbing layer. In this photodetector, the conductive layer is made of InGaAsP and transmits part of the incident light with a particular wavelength.
In the photodetector according to a second aspect of the present invention, the barrier layer is formed between the conductive and light-absorbing layers. Accordingly, this photodetector can receive and sense light that has been incident through the backside thereof. In addition, the conductive layer, made of InGaAsP, can selectively transmit light with a particular wavelength out of the incoming light. Accordingly, where the incoming light has two wavelengths of 1.3 xcexcm and 1.55 xcexcm, the photodetector may sense part of the incoming light with the latter wavelength of 1.55 xcexcm. In this manner, a high-performance photodetector with an optical filtering function is realized.
In one embodiment of the present invention, InGaAsP as a material for the conductive layer may have an absorption edge longer than 1.3 xcexcm and shorter than 1.55 xcexcm.
More specifically, the absorption edge is preferably longer than 1.35 xcexcm and shorter than 1.5 xcexcm.
In an alternative embodiment, InGaAsP as a material for the conductive layer may also have an absorption edge longer than 0.93 xcexcm and shorter than 1.3 xcexcm.
More particularly, the absorption edge is preferably longer than 0.93 xcexcm and shorter than 1.25 xcexcm.
In another embodiment of the present invention, the substrate and the conductive, barrier, light-absorbing and wide bandgap layers may be made of InP, InGaAsP, InP, InGaAs and InP, respectively.
In still another embodiment, the photodetector may sense light that has been incident on the photodetector through a backside of the substrate.
In yet another embodiment, a semiconductor multilayer structure, including the semiconductor conductive, carrier barrier, light-absorbing and wide bandgap layers, may have been formed on said surface region of the substrate. A second semiconductor conductive layer may have been formed on another surface region of the substrate and may be electrically isolated from the conductive layer included in the multilayer structure. A pad for use to electrically connect the photodetector to an external unit may have been formed on the second conductive layer. And the pad may be electrically connected to the doped region that has been defined in said part of the wide bandgap layer in the multilayer structure.
Still another inventive photodetector includes semi-insulating semiconductor substrate, semiconductor conductive layer, light-absorbing layer, wide bandgap layer, doped region and electrode. The conductive layer has been formed on a surface region of the substrate and has electrical conductivity. The light-absorbing layer has been formed on the conductive layer and absorbs light that has been incident on the photodetector. The wide bandgap layer has been formed on the light-absorbing layer and has a bandgap wider than that of the light-absorbing layer. The doped region has been defined in the wide bandgap layer by doping part of the wide bandgap layer with a dopant that reaches the light-absorbing layer. And the electrode has been formed on the doped region. In this photodetector, a semiconductor multilayer structure, including the semiconductor conductive, light-absorbing and wide bandgap layers, has been formed on said surface region of the substrate. A second semiconductor conductive layer has been formed on another surface region of the substrate and is electrically isolated from the conductive layer included in the multilayer structure. A pad for use to electrically connect the photodetector to an external unit has been formed on the second conductive layer. The multilayer structure is covered with an insulating film. An interconnect has been formed on the insulating film to electrical connect the electrode and the pad together. And the insulating film is a stack of an SiN layer and an SiO2 layer that has been deposited on the SiN layer.
In the photodetector according to a third aspect of the present invention, the insulating film, formed on the surface of the multilayer structure (i.e., photodiode mesa), is a stack of SiN and SiO2 layers. Accordingly, the interconnect capacitance, formed between the interconnect on the insulating film and the multilayer structure, can be reduced compared to a structure in which the insulating film is made of a single SiN layer. As a result, a high-speed photodetector with a reduced interconnect capacitance is realized.
In one embodiment of the present invention, the SiN layer may have a thickness of 20 nm through 100 nm, and the SiO2 layer may have a thickness of 400 nm or more.
In another embodiment of the present invention, the photodetector may further include a carrier barrier layer between the conductive and light-absorbing layers. The barrier layer prevents carriers, created in the conductive layer, from diffusing and entering the light-absorbing layer.
An inventive method for fabricating a photodetector includes the step of a) stacking semiconductor conductive, light-absorbing and wide bandgap layers in this order on a semi-insulating semiconductor substrate by a crystal growth process. The conductive layer has etch susceptibility different from that of the substrate. The light-absorbing layer absorbs incoming light. And the wide bandgap layer has a bandgap wider than that of the light-absorbing layer. The method further includes the steps of b) defining a doped region in part of the wide bandgap layer by doping said part with a dopant that reaches the light-absorbing layer; c) etching and patterning the wide bandgap and light-absorbing layers into respectively predetermined shapes; d) defining an etch mask on the conductive layer so that the wide bandgap and light-absorbing layers in the predetermined shapes are covered with the mask; and e) selectively removing part of the conductive layer using an etchant that etches said part of the conductive layer away with respect to the substrate.
In one embodiment of the present invention, the step c) may include the steps of: i) defining a first etch mask on the wide bandgap layer so that the doped region is covered with the first mask after the step b) has been performed; ii) selectively etching part of the wide bandgap layer away with respect to the light-absorbing layer; iii) defining a second etch mask on the light-absorbing layer so that the wide bandgap layer is covered with the second mask; and iv) selectively etching part of the light-absorbing layer away with respect to the conductive layer.
Specifically, the substrate and the conductive, light-absorbing and wide bandgap layers may be made of InP, InGaAsP, InGaAs and InP, respectively. And the etchant may contain hydrochloric acid.
In an alternative embodiment, the substrate and the conductive, light-absorbing and wide bandgap layers may be made of InP, InGaAsP, InGaAs and InP, respectively. And the steps ii) and iv) may be performed using an etchant containing sulfuric acid.
Another inventive method for fabricating a photodetector includes the step of a) stacking semiconductor conductive, carrier barrier, light-absorbing and wide bandgap layers in this order on a semi-insulating semiconductor substrate by a crystal growth process. The conductive layer has electrical conductivity. The carrier barrier layer prevents carriers, created in the conductive layer, from diffusing and entering upper layers thereof. The light-absorbing layer absorbs incoming light. And the wide bandgap layer has a bandgap wider than that of the light-absorbing layer. The method further includes the steps of: b) defining a doped region in part of the wide bandgap layer by doping said part with a dopant that reaches the light-absorbing layer; c) defining a first etch mask on the wide bandgap layer so that the doped region is covered with the first mask; d) selectively etching part of the wide bandgap layer away with respect to the light-absorbing layer using a first etchant; e) defining a second etch mask on the light-absorbing layer so that the wide bandgap layer is covered with the second mask; f) selectively etching part of the light-absorbing layer away with respect to the barrier layer using a second etchant; g) selectively etching part of the barrier layer away with respect to the conductive layer using a third etchant; h) defining a third etch mask on the conductive layer so that the wide bandgap, light-absorbing and carrier barrier layers are covered with the third mask; and i) selectively etching part of the conductive layer away with respect to the substrate using a fourth etchant.
In one embodiment of the present invention, the substrate and the conductive, barrier, light-absorbing and wide bandgap layers may be made of InP, InGaAsP, InP, InGaAs and InP, respectively. The first and third etchants may contain hydrochloric acid, while the second and fourth etchants may contain sulfuric acid.
Still another inventive method for fabricating a photodetector includes the step of a) stacking semiconductor conductive, light-absorbing and wide bandgap layers in this order on a semi-insulating semiconductor substrate by a crystal growth process. The conductive layer has etch susceptibility different from that of the substrate. The light-absorbing layer absorbs incoming light. And the wide bandgap layer has a bandgap wider than that of the light-absorbing layer. The method further includes the steps of b) defining a doped region in part of the wide bandgap layer by doping said part with a dopant that reaches the light-absorbing layer; c) etching and patterning the wide bandgap and light-absorbing layers into respectively predetermined shapes; d) selectively etching part of the conductive layer away, thereby defining a semiconductor multilayer structure, which includes the wide bandgap and light-absorbing layers in the predetermined shapes and the conductive layer, and leaving a second part of the conductive layer so that the second part serves as a second semiconductor conductive layer spaced apart from the conductive layer included in the multilayer structure; e) depositing SiN and SiO2 layers in this order over the surface of the multilayer structure, exposed parts of the substrate and the second conductive layer, thereby forming an insulating film including the SiN and SiO2 layers; f) removing part of the insulating film, which is located over the doped region in the wide bandgap layer included in the multilayer structure, thereby forming an opening over the doped region; g) forming an electrode on part of the doped region inside the opening; h) forming a pad for use to electrically connect the photodetector to an external unit on either part of the insulating film that has been formed on the exposed part of the substrate or another part of the insulating film that has been formed over the second conductive layer; and i) forming an interconnect on the insulating film to electrically connect the electrode and the pad together.
In one embodiment of the present invention, the steps g), h) and i) may be performed as a single process step.
In this particular embodiment, the single process step preferably includes: depositing a spacer film of SiN on the insulating film; defining a negative photoresist pattern on the spacer film to form the electrode, the pad and the interconnect; etching parts of the spacer film away using the photoresist pattern as a mask; depositing a metal on exposed parts of the insulating film and on the photoresist pattern, thereby forming a metal thin film thereon; and lifting the photoresist pattern off along with excessive parts of the metal on the photoresist pattern, thereby forming the electrode, the pad and the interconnect.
In an inventive photodetector, a semiconductor conductive layer has etch susceptibility different from that of a semiconductor substrate. Accordingly, a semiconductor multi-layer structure, including the conductive layer, does not increase its height too much. As a result, an interconnect can be formed easily and the photodetector of this type is mass-producible.
In another inventive photodetector, a carrier barrier layer is further formed between semiconductor conductive and light-absorbing layers. Accordingly, this photodetector can sense light that has been incident through the backside thereof. In addition, the conductive layer can selectively transmit incoming light with a particular wavelength. As a result, a high-performance photodetector with an optical filtering function (i.e., wavelength selectivity) is realized.
In a third inventive photodetector, an insulating film, deposited on the surface of a semiconductor multilayer structure, is a stack of SiN and SiO2 layers. As a result, a high-speed photodetector with a reduced interconnect capacitance is realized.